Within the context of manufacturing the sheet of paper through coating, an aqueous composition known as paper coating is deposited onto the surface of the substrate paper, its function being to give said sheet a certain number of properties, such as opacity, brightness, whiteness, or printability when using offset or heliography printing methods.
These formulations are made up of water, one or more mineral loads, one or more binders whether water-soluble or not, as well as various additives such as dispersing agents, water-retaining agents, optical brighteners, rheology modifiers, etc.
Within the coating's rheology, it is first and foremost important to have a product that is easy to pump and filter within the supply systems of the application process, and which has no tendency to form foam or splashes, nor excessively fast sedimentation. This requirement corresponds to increasing the low shear rate viscosity, or the Brookfield™ viscosity measured at 100 revolutions/minute and at 25° C. with the device of the same name, without which the paper coating is too liquid.
Another major rheological characteristic is the high shear rate viscosity, as expressed through a ACAV viscosity value at 25° C. measured in a capillary viscosimeter in which the coating may be subjected to high shear rates (105 at 3×106 s−1) of the same order of magnitude as those observed during the coating method when the coating blade scraping off the excess deposited paper coating is applied. High shear rate viscosity is a determining factor in which blade pressure to apply. The greater this viscosity is, the higher the blade pressure needs to be in order to control the weight of the deposited layer.
Increasing the dry solids content of coatings and coating speeds is a trend which has been observed in recent years, as it generates economic and/or qualitative advantages. However, this increase in dry solids content leads to an increase in the shear viscosity, and consequently to a rise in the necessary blade pressures. Higher coating speeds, meanwhile, cause an increase in the hydraulic force on the blade, and therefore on the pressure to exert, which may lead to overflows of coatings, known by the terms “bleedings” or “pearls.” This issue is recounted in the document WO 84/04491.
This dual requirement is joined by the need to reduce the phenomenon of water and water-soluble species migrating through the paper. This migration is sought to be reduced as much as possible, in order to prevent a rheological change in the coating left unused and recycled during the application process. This is referred to as a water retention phenomenon, which is sought to be improved, i.e. increased.
In terms of rheology modifiers and water-retaining agents, in recent years a particular class of comb-branched polymers having a (meth)acrylic skeleton with hydroxy- or methoxy-polyalkylene glycol side-chains potentially containing a hydrophobic monomer such as ethyl or butyl acrylate. These are aqueous solutions of water-soluble polymers, particularly such as those developed by the company COATEX™ through its range of Rheocarb™ products in the field of paper.
Several patents currently describe the application properties of these structures in a paper coating: WO 01/96007 A1, WO 04/044022 A1, WO 04/041883 A1, WO 07/069037 A1 and WO 08/149226 A1. These polymers may be introduced into the paper coating by means of the suspension of mineral materials whose rheology they improve (WO 01/96007 A1). Besides their ability to increase the coating's Brookfield™ viscosity, they make it possible to improve its optical whiteness (WO 04/044022 A1) and brightness (WO 04/041883 A1). They are also known for increasing not only the water-retention of the coating (WO 07/069037 A1), but also their high shear gradient viscosity (WO 08/149226 A1).
In these documents:                the hydrophobic ethyl acrylate or styrene monomer is always optional (particularly as indicated in WO 01/96007 A1, WO 04/044022 A1, WO 04/041883 A1 and WO 07/069037 A1);        whenever it is present, its concentration by weight is always less than 20% of the total weight of the monomers involved (5% by weight of ethyl acrylate in test #13 of document WO 04/044022 and 4.5% to 19.5% by weight of ethyl acrylate in tests #4 to 7 of document WO 01/96007 A1);        no mention is made of the possible role played by such a monomer; additionally, nothing could draw attention to it, as the 5 products mentioned above appear anecdotally among the 225 tests in said documents taken together;        the polymer which is the object of the corresponding inventions is always described as water-soluble;        the examples of document WO 08/149226 A1 show an increase in the ACAV viscosity of the paper coatings, which is assumed to make the corresponding polymers unsuitable for coatings with a high solid content and/or application speed.        